1. Field of the Invention
The present invention relates to an EL (electro luminescent) element drive circuit and an organic EL display device using the same drive circuit and, in particular, the present invention relates to an organic EL display device suitable for high luminance color display, with which display luminance of organic EL elements can be improved and white balance on a display screen of an electronic device such as a portable telephone set or a PHS, etc., can be easily regulated by regulating luminous intensities of R (red), G (green) and B (blue) display colors even when dynamic range of regulation of reference current value of each of R, G and B colors is small.
2. Description of the Prior Art
An organic EL display panel of an organic EL display device, which is mounted on a portable telephone set, a PHS, a DVD player or a PDA (personal digital assistance) and includes 396 (132×3) terminal pins for column lines and 162 terminal pins for row lines, has been proposed and the number of column lines and the number of row lines of such organic EL display panel tend to be further increased.
An output stage of a current drive circuit of such organic EL display panel includes an output circuit constructed with, for example, current-mirror circuits, which are provided correspondingly to respective terminal pins of the panel, regardless of the type of drive current, the passive matrix type or the active matrix type. Incidentally, in a case of the passive matrix type current-drive system, a drive current having a peak value is utilized in order to emit light earlier by initially charging an organic EL element having capacitive load characteristics, at a start time of light emission period. Particularly, in order to prevent variation of luminous intensities of R, G and B display colors for the passive matrix type, a reset period is provided after the current drive period to discharge residual charge of an organic EL element to be current-driven next to a predetermined constant voltage (for example, several volts) or to ground potential. In this manner, the drive current waveform and the peak current value and waveform thereof are not changed when the organic EL element is current-driven by the peak current generated after the resetting.
Incidentally, JPH9-232074A discloses a drive circuit for organic EL elements, in which the organic EL elements arranged in a matrix are current-driven and a terminal voltage of each organic EL element is reset by grounding an anode and a cathode of the organic EL element. Further, JP2001-143867A discloses a technique with which power consumption of an organic EL display device is reduced by current-driving organic EL elements with using DC-DC converters.
One of the problems of a conventional organic EL display device is that the predetermined reset period is necessary and luminous intensity is degraded since light emitting period is shortened when scan frequency is increased. Particularly, the residual charge must be discharged to the predetermined constant voltage after light emission of the organic EL element is ended and, in order to match the reset period to the longest discharge period of one of R, G and B display colors, the reset period becomes long by all means. On the other hand, when the resetting of the organic EL element is performed by discharging residual charge thereof to ground, it is possible to shorten the discharge period. However, a time period in which the potential of the organic EL elements must be increased from ground potential to the peak current becomes long. Therefore, the substantial light emission period of the organic EL element is shortened, resulting in degradation of luminous intensities thereof.
Another problem of the conventional organic EL display device is that, when the voltage drive is used to drive terminal pins thereof as in a liquid crystal display device, a display control becomes difficult and luminance variation becomes conspicuous due to difference in luminous sensitivity between R, G and B display colors. For this reason, the organic EL display device has to be current-driven. However, even when the current drive is employed, ratio of light emission efficiency for drive currents of R. G and B colors is, for example, R:G:B=6:11:10, which depends upon luminescent materials of the organic EL elements.
In view of this, it is necessary, in a current-drive circuit of an organic EL color display device, that white balance is obtained on a display screen thereof by regulating luminous intensity of each of R, G and B colors according to luminescent materials of EL elements for respective R, G and B colors. In order to realize such white balance regulation, regulation circuits for regulating luminous intensities of respective R, G and B colors on the display screen are provided.
It is usual, in the conventional organic EL display device, that the current-drive circuit of the organic EL display device generates drive currents for driving organic EL elements, which are connected to respective column line pins, by amplifying reference currents for R, G and B display colors and that the regulation of the drive currents for obtaining white balance is performed by regulating the reference currents for respective R, G and B display colors.
In order to regulate the reference currents for respective R, G and B colors, each of reference current generator circuits of a conventional drive current regulator circuit includes a D/A converter circuit of, for example, 4 bits and the reference currents for respective R, G and B display colors are regulated by setting a predetermined bit data for each of R, G and B display colors at 5 μA intervals within a range, for example, from 30 μA to 75 μA. With the fact that various organic EL materials have been developed recently, the luminance regulation for realizing white balance, which is realizable by the D/A converter circuits, is not enough since the dynamic range of regulation is as rough as 4 bits.
However, if the number of bits of the D/A converter circuit for luminance regulation of each of R, G and B display colors is increased to 6 to 8 bits, the size of the current drive circuit becomes large since the D/A converter circuits must be provided for respective R, G and B display colors, and, therefore, it becomes difficult to integrate the current drive circuit in one chip. Further, it is difficult to respond to the request of miniaturization of the display device portion.